Update broken migration link

[jessiehuff]

There is a broken link that is accessible through this page (you can refer to this migration guide text).

It is currently trying to navigate to https://github.com/patternfly/react-component-groups/migration.md and should be navigating to https://github.com/patternfly/react-component-groups/blob/main/migration.md

[github-actions]

🎉 This issue has been resolved in version 5.1.0-prerelease.6 🎉

The release is available on:

• GitHub release
• npm package (@prerelease dist-tag)

Your semantic-release bot 📦🚀